The present invention relates to an optical fiber cable for communication, having graded index type plastic optical fibers (hereinafter referred to as GI-POFs) and a resin cable body confining the GI-POFs.
Presently, optical fibers have been commonly employed to transmit a large volume of information at a high speed with reliability in the communication field.
The optical fibers include silica optical fibers, such as silica single-mode optical fibers, and resin optical fibers (plastic optical fibers). In particular, the plastic optical fibers have a larger diameter than the silica single-mode optical fibers and are excellent in flexibility, and accordingly, they are excellent in workability and safety at the time of end treatment and connection treatment during the installation of optical cables, and are useful.
Particularly, GI-POFs have high-speed large-volume transmitting capability and are expected to be optical fibers for the next generation communication.
The GI-POF is a plastic optical fiber, which is made to have an index distribution in the sectional direction. Namely, the GI-POF is constituted to have a refractive index distribution wherein the refractive index is high at the center in the sectional direction and decreases gradually, and accordingly, the light proceeding in the longitudinal direction in the GI-POF is concentrated in the vicinity of the center of the GI-POF by the effect of the refractive index. The high-speed large-volume transmitting capability is thereby achieved.
Thus, the high-speed large-volume transmitting capability of the GI-POF largely depends on the refractive index distribution, and in order to secure the transmitting capability of the Gi-POF, it is important to maintain the refractive index distribution to be a predetermined distribution.
Meanwhile, the production of an optical fiber cable of GI-POFs is carried out by covering and molding GI-POFs by extruding them together with a structural element, such as a tension member, for protection against tension, with e.g. a thermoplastic resin. During this covering and molding step, GI-POFs are likely to be affected by a heat of e.g. the thermoplastic resin melted at a high temperature. Accordingly, the physical properties of GI-POFs are likely to be deteriorated by the heat. Therefore, it is necessary to produce the cable so as not to be affected by the heat. As a typical method for producing a GI-POF, there is a method wherein a low molecular weight compound material having a different refractive index, is thermally diffused in a resin material to form a refractive index distribution to obtain a GI-POF.
With such a GI-POF, there is a possibility that due to the effect of the heat at the time of covering and molding to form a cable, the low molecular weight compound material is thermally diffused in the GI-POF, and the refractive index distribution is changed.
For example, in JP-A-11-211954, in order to prevent from increasing attenuation of GI-POF by the heat of melted covering resin material, a resin capable of being extruded at a relatively low temperature, such as polyethylene, is preliminarily extruded to cover the surface of GI-POFs and molded by means of a draw-down, whereby the GI-POFs are primarily covered and so-called jacket fibers are obtained. Thereafter, they are secondarily covered and molded by extruding them together with a structural element, such as a tension member, to produce an optical fiber cable of GI-POFs.
The structure of the optical fiber cable produced by such a method comprises, as an optical fiber cable 50 illustrated in FIG. 4(a), GI-POFs 51a and 51b, a primary covering layer 56 made of e.g. polyethylene, and a secondary covering layer 53 being the resin cable body made of e.g. a thermo-plastic resin extruded to form the outermost layer.
Further, on the other hand, JP-U-60-60714 and JP-A-7-72356 propose a structure provided with a spacing between a covering resin material being the resin cable body and optical fibers, as illustrated in FIG. 4(b), or a structure provided with a spacing between a jacket fiber and a covering resin material as illustrated in FIG. 4(c). In FIG. 4(b), an optical fiber cable 60 is constituted by tension members 62a and 62b, a covering resin material 63 being the resin cable body, and a spacing 64 in which two optical fibers 67a and 67b are arranged. An optical fiber cable 70 illustrated in FIG. 4(c), is constituted by a covering layer 73 being the resin cable body made of e.g. a thermo-plastic resin extruded to form the outermost layer, a spacing 74 provided in the covering layer 73, and an optical fiber 78 arranged in the spacing and primarily covered with a covering layer 76.
However, the optical fiber cable of GI-POFs obtained by the production method described in JP-A-11-211954, has had the following problem:
Namely, in a jacket fiber which is a GI-POF smaller than 1 mm in diameter primarily covered with e.g. polyethylene, there has been a thermal durability problem such that in a high temperature durability test (at 70xc2x0 C. for 24 hours), as the resin such as polyethylene being the covering material, is heat-shrunk, microbents are formed on the surface of the GI-POF, and consequently the attenuation is increased.
Further, in the optical fiber cable of GI-POFs having a structure wherein a spacing between the covering resin material and the optical fibers is provided as shown in JP-U-60-60714, a plurality of GI-POFs are present in a single hole, and there has been a problem in pressure resistance such that, when an external force is applied to the cable e.g. when it is stepped by e.g. a person, the plurality of optical fibers in the single hole are brought in contact with each other, pressed each other, and in the worst case, squashed each other or plastically deformed to increase the attenuation.
Further, in JP-A-7-72356, the increase of the attenuation which usually occurs due to flexing action at the time of bending, can be suppressed by making the unoccupied ratio of the optical fiber cable to be from 2 to 30%. However, the upper limit of the unoccupied ratio is limited from the viewpoint of easiness of attaching an optical connector when the optical connector is attached to the optical fiber cable. And so, in a GI-POF for which a high-speed large-volume transmission capacity is required, there has been a problem in the mechanical property such that the increase of the attenuation due to flexing action at the time of bending, can not be suppressed to zero.
In order to solve the above problems, it is an object of the present invention to provide an optical fiber cable having a plurality of GI-POFs and a resin cable body confining the GI-POFs, and being excellent in the thermal durability, pressure resisting property and flexural mechanical property, whereby the attenuation does not increase.
The present invention provides an optical fiber cable having a plurality of GI-POFs and a resin cable body confining the GI-POFs, wherein the resin cable body has holes as many as the number of the GI-POFs, extending longitudinally therethrough, and the GI-POFs are distributed and arranged one by one in the holes so that they are freely movable in the respective holes.
Here, the movable range of the GI-POFs in the holes, is preferably at least twice as large as the diameter of the GI-POFs. The wall thickness of the resin cable formed by the plurality of holes, is preferably equivalent or larger than the diameter of the GI-POFs. And the wall thickness of the resin cable is preferably at least 0.5 mm.
Further, the plurality of holes are arranged in parallel, and the wall thickness at the central portion of the resin cable body, is preferably thicker than the wall thickness at both ends of the holes arranged in parallel and located at both ends. The hardness of the resin cable body is at most 50 by Shore D hardness.
Further, the GI-POFs are preferably of a perfluorinated type or a polymethyl methacrylate (PMMA) type. A tension member is preferably embedded in the resin cable body.